Spectral imaging provides three-dimensional information about a subject, two dimensions being dedicated to the two spatial dimensions of the image and the third dimension comprising the spectral information. Hyperspectral imaging is a term dedicated to spectral imaging where the spectral dimension is better resolved than usual filter-based techniques, with, typically, a high number of spectral bands (100 bands or more in the visible domain) and a fine resolution (10 nm or less in the visible domain).
Spectral imaging can be performed in different ways including utilising Fourier transform based technology. Fourier transform spectral imaging requires interferogram acquisition. A plurality of physical interferometric configurations is suitable for the purpose of interferogram acquisition including Michelson interferometry, which tends to be mainly used for wavelengths in the infrared region. In addition, Mach Zender, Sagnac and polarisation interferometers can also be used.
U.S. Pat. No. 5,781,293 describes a Fourier transform spectrometer in which birefringent prisms are used to introduce the path difference between two light polarisations and a Fourier transform of the resulting interferogram at a detector that provides the spectral distribution of the incident light.
In an article by A. R. Harvey and D. W. Fletcher-Holmes titled “Birefringent Fourier-transform imaging spectrometer,” Optics Express, vol. 12(22), pp. 5368-74 (2004), a Fourier transform imaging spectrometer is disclosed which comprises a birefringent interferometer utilising a matched pair of Wollaston prisms, the interferogram being produced by movement of one element in the interferometer.
A Wollaston prism is one of the possible configurations for an assembly of birefringent elements for separating polarised radiation into two orthogonally polarised components.
Whilst these configurations can be extremely compact as they provide common path type interferometers without having to use additional beam splitting elements, they tend to suffer from disadvantages. One disadvantage is economical because of the complexity of manufacturing paired Wollaston prisms. The first Wollaston prism splits the incident radiation into two polarisation components in different directions and the second Wollaston prism re-directs the two polarisation components so that they are parallel. For this configuration to work appropriately, the two Wollaston prisms must be paired and well aligned. The pairing of the Wollaston prisms is performed during production by ensuring that the wedge or split angle of the first Wollaston prism is equal to the wedge or split angle of the second Wollaston prism.
Another disadvantage relates to the field dependence of the optical path length difference, and in particular, the field dependence of the relation between the optical path length difference and the translation of the second Wollaston prism. For imaging systems where the field of view is not restricted to a single point, the principal rays of several field points arise with different incidence angles on the Wollaston prism, and the optical path length difference is thus also dependent on the incidence angle. The spectrum recovery for the different field points then must take into account this field dependence of the relation between the optical path length difference and the prism translation. For accurate spectral measurements, the same signal processing cannot be applied to all field points and therefore signal processing is more complex.
In addition, Fourier transform spectral imaging may be time consuming if many measurements need to be made and processed. Such measurements may also be disturbed by air movement.